I understand, now, that "Sigil used to leave them alone" probably isn't exactly true. It's been a while. I'm guessing Sigil just made them all point to the generic resource-type folder structure Sigil always enforced. Like I said; I understand if this is just a consequence of Sigil 1.0.0. I'm not looking to reinvent the wheel.
Essentially ... Sigil has
always changed broken links into different broken links upon import. It's just that broken links that happened to follow Sigil's old ../Text/, ../Images/, ../Styles/, pattern
appeared to be left alone.